Throughout this application various publications, patents, and published patent applications are referred to by an identifying citation. The disclosure of the publications, patents, and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
Numerous methods are known for the purification of organic solids, as for example described in Chemical Separation and Measurement, Chapters 14 to 17, (1974), W. B.
Saunders Company, by Peters et al., and in Purification of Laboratory Chemicals, 3.sup.rd Edition, (1988), Butterworth Heiriemann, by Perin et al. Of those methods described, crystallization has been widely used and has proven to be an effective method for the purification of solids. Crystallization can be from a single solvent or from a mixture of solvents. Other methods for the purification of solids include: sublimation; zone refining as described in Zone Melting of Organic Compounds, (1963), Wiley, by Herngton; and, chromatographic separation techniques, such as, column chromatography, liquid chromatography, gas chromatography, gel permeation chromatography and ion exchange chromatography. For ionic materials electrophoresis can also be used. The most appropriate method for the purification of a particular material is not always apparent without experimentation, and is dependent upon many different factors. These may include the physical properties of the material itself, such as melting point; polarity; the type and amount of impurity present in the material to be purified; and how similar or different the impurity is from the desired pure material. Other important considerations include the purity desired in the material for its particular use and the cost or complexity of the purification technique.
A limitation of crystallization as a method of purification includes the formation of solvate complexes in which solvent molecules are tightly held in the crystallized material. Likewise, the impurities in a material may co-crystallize with the material so that little if any purification is achieved. Sublimation is not applicable to many solids because the solids are not stable at the temperatures required for volatilization or they may become liquid at that temperature, and while chromatography is particularly well suited to small scale separations, such as needed for analysis, it is severely limited for large scale purifications. Furthermore, chromatographic techniques invariably use a diluent as a carrier, either a liquid or gas. In the case of liquids, these must be readily removed from the material after the impurity or impurities have been separated in the chromatographic process. For gas chromatography to be used, the material to be purified must have significant volatility and good thermal stability, which excludes its use for many types of compounds especially those of high molecular weight, with high boiling points, or with ionic salt properties.
It is critical for many ionic materials that they be absolutely free of impurities in their intended use. For example, materials used in applications where optical and electrical properties are important must be especially pure, as described, for example, by Sandman in J. Crystal Growth, 1988, 89, 111-116. Similarly, Stoffel et al. in U.S. Pat. No. 4,994,110 describe the negative impact of impurities, at low levels, in lithium salts of anionic dyes used in ink jet inks and describe an ion exchange process for impurity removal.
Purification of ionic salts is especially difficult in that the number of methods applicable is limited mainly to crystallization and particularly to the use of polar solvents. These solvents tend to associate strongly with the ionic material making it difficult to obtain solvent-free solids in many cases
Similarly, the purity of electrolytes is critical to the performance of electrochemical cells, batteries and related electrochemical devices. While the purification of electrolyte solvents may be performed by standard methods for the purification of liquids, such as distillation, the ionic solutes, especially organic ionic solutes, are often not readily purified by such methods. Other approaches have therefore been used for the purification of ionic solutes for use in electrolytes for electrochemical cells and battery applications.
For example, U.S. Pat. No. 4,895,778 to Nalewajek describes a procedure for the removal of impurities from electrolytes, particularly metal impurities, by the use of a chelating resin. U.S. Pat. No. 4,308,324 to Newman describes a two step procedure for handling contamination in electrolytes. In this process the electrolyte is treated in the first step with a mercury/alkaline metal amalgam, followed by treatment with an oxidizing agent. In another approach, Laverdure et al., Proceedings of the Symposium on Primary and Secondary Ambient Temperature Lithium Batteries, Electrochemical Soc., 1988, 692-698, describe the use of alumina, lithium-mercury amalgam, sodium-potassium alloy, or lithium foil to remove impurities from electrolytes such as lithium hexafluoroarsenate in 2-methyltetrahydrofuran.
In an alternative approach to high purity electrolytes, the ionic electrolyte solute is prepared in presence of complexing solvent. For example, U.S. Pat. No. 4,880,714 to Bowden describes a method of preparation of LiPF.sub.6 electrolyte salt in presence of a complexing ether solvent and isolation of the LiPF.sub.6 ether complex. The solvate complex is reported to be stable and protects the ionic salt from undesirable decomposition. In a related example, U.S. Pat. No. 4,321,314 to Bowden et al. reports the formation of a very stable molecular complex between LiClO.sub.4 and dimethoxyethane. This complex is reported to yield electrolyte solutions which contain no free dimethoxyethane.
Of those salts used as ionic solutes in electrolytes, the fluoroalkylsulfonylimides have proved to be particularly difficult to obtain in pure form, and in particular the lithium salts of bis(perfluoromethylsulfonyl)imide, bis(perfluoropropanesulfonyl)imide and bis(perfluorobutanesulfonyl)imide.
For example, U.S. Pat. No. 5,652,072 to Lamanna et al. reports the presence of impurities in ionic organic electrolyte salts, lithium perfluoroalkylsulfonylimides, but offers no purification process. Dominey in a report "Novel Stable, Non-Complexing Anions for Rechargeable Lithium Batteries", NTIS PB 93-138741 describes unsuccessful attempts to purify lithium bis(trifluoromethylsulfonyl)imide. In particular, the inability to obtain purification by recrystallization of the lithium bis(trifluoromethylsulfonyl)imide from dioxane is reported. Choquette et al. in J. Electrochem. Soc., 1998, 3500-3507, report that attempts to purify this same lithium imide by recrystallization were unsuccessful. Japanese Patent Publication No. 09-255685, published Sep. 30, 1997, to Suzuki et al., describes a purification of perfluoroalkylsulfonylimides by crystallization from dioxane followed by dissolution of the crystals in a polar solvent of boiling point less than 100.degree. C. After filtration to remove traces of insoluble material, the solution in the polar solvent is evaporated to recover the sulfonylimide. This process reduced the levels of sodium, potassium, calcium magnesium and sulfate impurities, however, the recovery of the pure salt was reported to be only 60%.
To be useful any purification process for the purification of ionic organic salts needs to meet a number of criteria, including: the ability to handle large scale purifications; provide high yield; handle a variety of impurities; employ standard equipment; and be applicable to several families of materials.
It is apparent that the purification methods described above meet only some of these criteria: each process fails to meet at least one criterion. There remains, therefore, a need for improved methods of purification to handle a range of impurity types in ionic organic salts especially those used in electrolytes. The need is particularly important for ionic organic salts such as perfluoroalkylsulfonylimide alkali metal salts, for example bis(trifluoromethylsulfonyl)imide lithium salt, which because of their involved manufacturing processes may contain several types of impurity, and which are needed in high purity in applications such as electrolyte salts for the electrolytes of electrochemical cells.